How does a physician treat her/his ppo patients differently than her/his hmo patients?

Very frustratingly: HMOs generally cause more frustration by essentially preventing patients from seeing certain doctors, from going to certain hospitals, or using certain medications. There are big costs and aggravations for going out-of-network. So, a patient should buy the insurance that allows him or her the most freedom of choice (which does usually cost a bit more, but this is America, and Freedom Is Not Free).